Method for preparng capsules comprising at least one water-soluble or hydrophilic substance and capsules obtained therefrom

ABSTRACT

The present invention relates to a method for preparing solid microcapsules that comprises of the following steps of preparing a composition C1, comprising at least one water-soluble or hydrophilic substance dispersed in a hydrophobic phase; adding, under agitation, of the said composition C1 in a polymeric composition C2, the compositions C1 and C2 being immiscible with each other, whereby an emulsion (E1) is obtained; adding, under agitation, of the emulsion (E1) in a composition C3, the compositions C2 and C3 being immiscible with each other, whereby a double emulsion (E2) is obtained; shearing the emulsion (E2), whereby a double emulsion (E3) is obtained; and polymerizing the composition C2, wherein solid microcapsules dispersed in the composition C3 are obtained.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 U.S.C. § 371 of International Patent Application No. PCT/EP2018/057028, filed Mar. 20, 2018, which claims priority of French Patent Application No. 17 52331, filed Mar. 21, 2017. The entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The object of the present invention relates to a preparation method for preparing capsules comprising at least one water-soluble or hydrophilic substance. The invention also relates to the capsules as obtained as well as the compositions containing the capsules.

BACKGROUND

A number of highly water-soluble or hydrophilic substances are often added to formulated products in order to confer them with interesting beneficial application properties or to improve the performance thereof. In certain cases, these substances are fragile molecules that are susceptible to being degraded as a result of interactions with their environment by means of mechanisms such as hydrolysis, thermal denaturation or oxidation, which leads to a decline in the performance of the formulated product.

In other cases, these substances very significantly modify the ionic strength of the formulated product, which results in adverse consequences on the stability of the product that can for example undergo a rapid separation of phases.

In still other cases, these substances react with other components of the formulated product, which also results in adverse consequences on stability as well as a decline in performance levels.

The encapsulation of water-soluble or hydrophilic substances represents a technique of great beneficial interest for overcoming the limitations related to performance or stability of the formulated products that contain them.

A very large number of capsules have been developed in order to protect and/or isolate active ingredients in formulated products, and especially water-soluble or hydrophilic substances. These capsules are the results obtained from manufacturing methods such as spray-drying, interfacial polymerisation, interfacial precipitation, or solvent evaporation among many others. The enveloping shell of the capsules produced by these methods does not constitute a sufficiently sealed barrier that effectively protects against the diffusion of molecules that are as minuscule in size as the water-soluble or hydrophilic substances dissolved in their core or the chemical species that are responsible for the degradation of substances such as dioxygen or water. Thus, as a result these capsules induce the relatively rapid leaking of the water-soluble or hydrophilic substances contained therein, accompanied by the rapid degradation of these substances once the capsules are dispersed in the formulated product.

The difficult problem of preventing the diffusion of molecules that are as minuscule in size as the water-soluble or hydrophilic substances in question or the chemical species that are responsible for the degradation of the substances mainly explains the fact that to date there do not exist capsules that possess sufficient protection and retention properties for satisfactorily protecting and retaining the water-soluble or hydrophilic substances.

SUMMARY

The present invention therefore serves the object of providing a method for encapsulating water-soluble or hydrophilic, or even very highly water-soluble or hydrophilic, substances, while also preventing the aforementioned contamination problems.

The present invention therefore also serves the object of providing an encapsulation method by means of double emulsion that makes it possible to obtain size controlled capsules with a size in particular of less than 20 μm, or even 5 μm.

The present invention also serves the object of providing capsules containing at least one water-soluble or hydrophilic, or even very highly water-soluble or hydrophilic, substance, that exhibit excellent retention capacity.

The present invention also serves the object of providing capsules containing at least one water-soluble or hydrophilic, or even very highly water-soluble or hydrophilic, substance, that present a plurality of protective barriers, in particular a steric barrier, a thermodynamic barrier, and a physical barrier.

Thus, the present invention concerns a method for preparing solid microcapsules comprising the following steps:

a) the preparation of a composition C1, comprising at least one water-soluble or hydrophilic substance dispersed in a hydrophobic phase;

b) the addition, under agitation, of said composition C1 into a polymeric composition C2, the compositions C1 and C2 being immiscible with each other;

the composition C2 comprising at least one monomer or polymer, at least one crosslinking agent, and optionally at least one photoinitiator or a crosslinking catalyst, the viscosity of the composition C2 being comprised between 500 mPa·s and 100,000 mPa·s at 25° C., and preferably being greater than the viscosity of the composition C1;

whereby an emulsion (E1) is obtained comprising droplets of the composition C1 dispersed in the composition C2;

c) the addition, under agitation, of the emulsion (E1) in a composition C3, the compositions C2 and C3 being immiscible with each other;

the viscosity of the composition C3 being comprised between 500 mPa·s and 100,000 mPa·s at 25° C., and preferably being greater than the viscosity of the emulsion (E1);

whereby a double emulsion (E2) is obtained comprising droplets dispersed in the composition C3;

d) the application of a shear to the emulsion (E2), whereby a double emulsion (E3) is obtained comprising size controlled droplets dispersed in the composition C3; and

e) the polymerisation of the composition C2, whereby solid microcapsules dispersed in the composition C3 are obtained.

The method of the invention therefore makes it possible to prepare solid microcapsules comprising a core and a solid enveloping shell that completely encapsulates at its periphery the core, in which the core is a composition C1 comprising at least one water-soluble or hydrophilic substance dispersed in a hydrophobic phase.

The microcapsules obtained according to the method of the invention advantageously provide the ability to protect the water-soluble or hydrophilic substances contained therein from being degraded by degradation inducing chemical species originating from the external environment of the capsules, in accordance with three different mechanisms:

diffusion to the interior of the capsule of chemical species causing degradation of the water-soluble or hydrophilic substances is advantageously limited by the rigid, high density crosslinked enveloping shell of the capsules;

the hydrophobic phase of the core constitutes an additional barrier for curbing the diffusion of hydrophilic degrading species to the water-soluble or hydrophilic substances; and

when the water-soluble or hydrophilic substances are in a solid state (ie for example in the form of particles or crystals as described here below), this state limits the amount of material that is in direct contact with the degrading species: only one exterior layer is found to be directly in contact with the hydrophobic phase which contains the water-soluble or hydrophilic substances. Given that the diffusion of chemical species to the interior of a solid is very significantly slowed down as compared to a liquid, the molecules of water-soluble or hydrophilic substances, situated at the centre of the particles, are found to be completely isolated from the degrading chemical species.

The capsules of the invention are particularly advantageous in that they present a plurality of retention barriers in order to achieve this level of performance:

a steric barrier: by virtue of their size, water-soluble or hydrophilic substances, in particular in solid form, in particular in the form of particles and/or in the form of a crystal, in particular of a salt, preferably having an average size of less than 1 μm, are not able to pass through the pores of the rigid polymer enveloping shell;

a thermodynamic barrier: the solubility of the water-soluble or hydrophilic substances in the hydrophobic phase of the core of the capsules is negligible, thus eliminating the possibility for molecules of water-soluble or hydrophilic substances, to dissolve in the core and subsequently to thus diffuse more rapidly to the exterior of the capsule; and

a physical barrier: the solid enveloping shell advantageously limits the diffusion of the dissolved chemical species to the exterior of the capsule.

DETAILED DESCRIPTION

The method of the invention consists in producing a double emulsion composed of droplets of a hydrophobic phase containing the water-soluble or hydrophilic substances, enveloped in a crosslinkable liquid phase. These double droplets are then rendered monodisperse in size before being converted by means of crosslinking or polymerisation into rigid capsules. The preparation involves 5 steps as described in greater detail here below.

Step a)

The step a) of the method according to the invention consists in preparing a composition C1 comprising at least one water-soluble or hydrophilic substance.

Composition C1

The composition C1 is a dispersion and comprises at least one water-soluble or hydrophilic substance, dispersed in a hydrophobic phase.

According to the invention, the term “water-soluble or hydrophilic substance” is used to refer to a substance having a solubility in water, measured at ambient temperature, for example T=25° C.±2° C., and at ambient pressure, for example 1 013 mbar, at least equal to 1 gram/liter (g/L) (obtaining of a macroscopically isotropic and transparent solution, whether or not coloured).

This water solubility is preferably greater than or equal to 10 g/L, in particular greater than or equal to 50 g/L, or indeed even greater than or equal to 100 g/L.

Moreover, the solubility of the water-soluble or hydrophilic substance in the hydrophobic phase is advantageously less than 20 g/L, preferably less than 10 g/L, and in a preferred manner less than 1 g/L.

According to one embodiment, the water-soluble or hydrophilic substance is in a solid state (or form) in the absence of water, at ambient temperature, for example T=25° C.±2° C., and at ambient pressure, for example 1 013 mbar, and in particular is in the form of particles and/or in the form of a crystal, in particular of a salt, preferably having an average size of less than 1 μm.

In the context of the present invention, the term “size” is used to refer to the diameter, in particular the mean diameter, of the particles and/or of the crystals.

According to one embodiment, among the water-soluble or hydrophilic substances, mention may be made of those selected from among the group constituted of the following compounds: ascorbic acid, in particular L-ascorbic acid, and the biologically compatible derivatives or salts thereof, such as for example, ethyl vitamin C; enzymes; antibiotics; tensor effect components; alpha-hydroxy acids and the salts thereof; hydroxylated polyacids; sucroses and the derivatives thereof; urea; amino acids; oligopeptides; water-soluble plant extracts, and yeasts; protein hydrolysates; hyaluronic acid; mucopolysaccharides; vitamins B1, B2, B3, B6, B12, H, PP; panthenol; folic acid; acetyl salicylic acid; allantoin; glycyrrhetic acid; kojic acid; hydroquinone; dihydroxyacetone (or DHA); EUK 134 (International Nomenclature of Cosmetic Ingredients (INCI) name: Ethylbisiminomethylguaiacol manganese chloride); organometallic or inorganometallic catalysts of platinum, palladium, titanium, moybdenum, copper or zinc; disinfecting and antibacterial agents and in particular silver salts; brightening or whitening/lightening agents, dispersibility modifiers, stabilisers, stain removing agents, in particular those listed in US patent documents U.S. Pat. No. 6,335,315 and U.S. Pat. No. 5,877,14; compounds that can be used as fire resisting agents or flame retarding agents, such as boron salts, aluminum hydrates or magnesium hydroxides, compounds that can be used as Phase Change Materials (PCMs) such as ammonium carbonate, ammonium chloride, cesium carbonate, cesium sulfate, calcium citrate, calcium chloride, calcium hydroxide, calcium phosphate, calcium saccharate, calcium sulfate, cerium phosphate, iron phosphate, lithium carbonate, lithium sulfate, magnesium chloride, magnesium sulfate, manganese chloride, manganese nitrate, manganese sulfate, potassium acetate, potassium carbonate, potassium chloride, potassium phosphate, rubidium carbonate, rubidium sulfate, disodium tetraborate, sodium acetate, sodium bicarbonate, sodium bisulfate, sodium citrate, sodium chloride, sodium hydroxide, sodium nitrate, sodium percarbonate, sodium persulfate, sodium phosphate, sodium propionate, sodium selenite, sodium silicate, sodium sulfate, sodium tellurate, sodium thiosulfate, strontium hydrophosphate, zinc acetate, zinc chloride, sodium thiosulfate, and polyethylene glycols.

A composition C1 according to the invention may advantageously comprise from 0.1% to 40%, preferably from 0.5% to 35%, in particular from 1% to 30%, better still from 5% to 25%, and even better still from 10% to 20%, by weight of water-soluble or hydrophilic substance(s) in relation to the total weight of the said composition C1.

According to one embodiment, for optimum protection, the hydrophobic phase of the composition C1 comprises oils having medium polarity at low interfacial tension with water.

According to one preferred embodiment, the hydrophobic phase of the composition C1 comprises at least one oil whose interfacial tension with water is comprised between 25 mN/m and 50 mN/m. The interfacial tension (or surface tension or superficial tension) is measured in accordance with conventional methods, in particular according to the pendant drop method or according to the Du Nouy ring method. The Du Nouy ring technique consists of dipping a platinum ring in the liquid the surface tension of which is to be measured and then exerting a pulling force on the ring towards the liquid/air interface at ambient temperature. The tensiometer will search for the maximum force at this interface and measure it (Figure K in the French patent application FR 3 004 641). This force is written as Pm and its value is not obtained directly; it undergoes a correction according to the geometry of the ring.

The device will first calculate gs* referred to as the measured surface tension. It is obtained by the relation: gs*=Pm/[2p(Ri+Ro)]; with Ri and Ro being respectively the inner and outer radii of the ring.

Then, it will deduce gs therefrom based on the relation: gs=gs*·F; where F is the correction factor depending on the density of the liquid as well as the ratio of the diameters of the ring. The relevant manipulation is carried out with a Krüss K12 tensiometer, the liquid is placed in a thoroughly cleaned crystalliser (crystalliser of 46 mm diameter ref GL5 Kruss) and the ring is made of platinum iridium (ref Kruss R101). The device gives the surface tension directly in mN/m.

Preferably, the oil is selected from among plant, animal or synthetic oils.

These oils may generally differ with respect to their chemical nature, such as: esters, ethers, alkanes, alcohols, alkenes, plant oils, oils of natural or synthetic origin, hydrocarbons or silicones, either linear or branched, whether or not polymerised.

Among these oils, mention may be made for example, of paraffin oil or polyalphaolefins.

When the water-soluble or hydrophilic substance is in a solid form, in particular in the form of particles and/or crystals, in particular having an average size of less than 1 μm, the composition C1 may be prepared in several ways.

A first technique consists in creating droplets of water containing the water-soluble or hydrophilic substance in dissolved form, and then evaporating the water.

Thus, according to a first embodiment, the composition C1 is prepared by dispersing, in the hydrophobic phase, at least one water-soluble or hydrophilic solid substance in the form of a powder, obtained in particular by atomisation.

The atomisation (spray-drying) makes it possible to obtain particles in powder form which is then dispersed in the hydrophobic phase which over time will eventually form the core of the microcapsules according to the invention.

According to another embodiment, the composition C1 is obtained by preparing a nanoemulsion of droplets of water in the hydrophobic phase. Nanoemulsions are a class of emulsions which generally have an average droplet size of less than or equal to 1 μm, preferentially less than or equal to 800 nm.

The nanoemulsion thus obtained will form the core of the capsules.

The nanoemulsion thus obtained can then be dried by agitation under reduced pressure and/or heating or by lyophilisation (freeze-drying). In the case of drying by agitation under reduced pressure, the nanoemulsion, stirred by means of a propeller stirrer or indeed a bar magnet set in motion by a magnetic stirrer, is placed in an enclosure whereof the pressure is brought by means of a vacuum pump to below 100 mbar, preferably below 50 mbar, even better below 10 mbar, thus allowing for the water from the droplets to evaporate. According to one variant of the method, the enclosure/chamber may be provided with a condensation column in order to maintain the vapour pressure of the water well below the saturation vapour pressure. In the case of drying by heating, the nanoemulsion, stirred by means of a propeller stirrer or indeed a bar magnet set in motion by a magnetic stirrer, is heated to a temperature comprised between 30° C. and 70° C., thus making it possible to evaporate the water from the droplets. According to one variant of the method, the drying processes under reduced pressure and by heating may be combined if necessary. In the case of drying by lyophilisation, the nanoemulsion is introduced into a lyophiliser. This method consists in freezing the nanoemulsion at a temperature preferably greater than the solidification temperature of the hydrophobic phase of the composition C1, and then removing the water by means of sublimation.

A second technique consists in grinding a coarse powder of at least one solid water-soluble or hydrophilic substance in order to obtain particles of the desired size and then dispersing them in the hydrophobic phase which will eventually form the core of the capsules.

Thus, according to yet another embodiment, the composition C1 is obtained by means of grinding at least one solid water-soluble or hydrophilic substance and then dispersing the said ground substance in the hydrophobic phase.

Step b)

The step b) of the method according to the invention consists in preparing a first emulsion (E1).

The first emulsion consists of a dispersion of droplets of the composition C1 in a polymeric composition C2 that is immiscible with C1, created by addition dropwise of C1 to C2 under agitation.

During the step b), the composition C1 is at a temperature of between 0° C. and 100° C., preferably between 10° C. and 80° C., and preferably between 15° C. and 60° C. During the step b), the composition C2 is at a temperature of between 0° C. and 100° C., preferably between 10° C. and 80° C., and preferably between 15° C. and 60° C.

Under the conditions of addition in step b), the compositions C1 and C2 are not miscible with each other, which signifies that the amount (by weight) of the composition C1 capable of being solubilised in the composition C2 is less than or equal to 5%, preferably less than 1%, and preferentially less than 0.5%, in relation to the total weight of the composition C2, and that the amount (by weight) of the composition C2 capable of being solubilised in the composition C1 is less than or equal to 5%, preferably less than 1%, and preferentially less than 0.5%, in relation to the total weight of the composition C1.

Thus, when the composition C1 comes into contact with the composition C2 under agitation, the latter is dispersed in the form of droplets, referred to as single droplets.

The composition C2 is agitated in a manner so as to form an emulsion comprising droplets of the composition C1 dispersed in the composition C2. This emulsion is also referred to as “single emulsion” or C1-in-C2 emulsion.

In order to implement the step b), use may be made of any type of agitator usually used to form emulsions, such as, for example, a mechanical agitator with paddles, a static emulsifier, an ultrasonic homogeniser, a membrane homogeniser, a high pressure homogeniser, a colloid mill, a high shear disperser or a high speed homogeniser.

The composition C1 is as defined above.

Composition C2

The composition C2 is intended for use in forming the future solid enveloping shell of the microcapsules.

The volume fraction of C1 in C2 may vary from 0.1 to 0.7 in order to control the thickness of the enveloping shell of the capsules obtained at the end of the method.

According to one embodiment, the ratio between the volume of composition C1 and the volume of composition C2 varies between 1:10 and 10:1. Preferably, this ratio is comprised between 1:3 and 5:1, preferably between 1:3 and 3:1.

Preferably, the viscosity of the composition C2 at 25° C. is comprised between 1000 mPa·s and 50,000 mPa·s, preferably between 2000 mPa·s and 25,000 mPa·s, and for example between 3000 mPa·s and 15,000 mPa·s.

Preferably, the viscosity of the composition C2 is greater than the viscosity of the composition C1.

The viscosity is measured by means of a rheometer, model Haake Rheostress™ 600 equipped with a cone having 60 mm diameter and 2 degrees cone angle, and a temperature control cell set at 25° C. The value of the viscosity is read for a shear rate equal to 10 s⁻¹.

According to this embodiment, the destabilisation kinetics of the droplets of the emulsion (E1) is significantly slow, which makes it possible for the enveloping shell of the microcapsules to be polymerised during the step e) before the emulsion is destabilised. The polymerisation, once completed, then provides a thermodynamic stabilisation. Thus, the relatively high viscosity of the composition C2 ensures the stability of the emulsion (E1) obtained at the conclusion of step b).

Preferably, the interfacial tension between the compositions C1 and C2 is low. Typically, these interfacial tensions vary between 0 mN/m and 50 mN/m, preferably between 0 mN/m and 20 mN/m.

The low interfacial tension between the compositions C1 and C2 also advantageously makes it possible to ensure the stability of the emulsion (E1) obtained at the conclusion of step b).

The composition C2 contains at least one monomer or polymer, at least one crosslinking agent, and optionally at least one photoinitiator, or crosslinking catalyst, thereby making it crosslinkable.

According to one embodiment, the composition C2 comprises from 50% to 99% by weight of monomer or polymer, or a mixture of monomers or polymers, in relation to the total weight of the composition C2.

According to one embodiment, the composition C2 comprises from 1% to 20% by weight of crosslinking agent or of a mixture of crosslinking agents, in relation to the total weight of the composition C2.

According to one embodiment, the composition C2 comprises from 0.1% to 5% by weight of photoinitiator or a mixture of photoinitiators, in relation to the total weight of the composition C2.

According to one embodiment, the composition C2 comprises from 0.001% to 70% by weight of crosslinking agent in relation to the weight of the said composition C2.

According to the invention, the term “monomer” or “polymer” refers to any base unit that is suitable for the formation of a solid material by polymerisation, either alone or in combination with other monomers or polymers.

These monomers may be selected from among monomers comprising at least one reactive functional group selected from among the group constituted of the functions: acrylate, methacrylate, vinyl ether, N-vinyl ether, mercaptoester, thiolene, siloxane, epoxy, oxetane, urethane, isocyanate, and peroxide.

In particular, the monomers may be selected from among monomers bearing at least one of the aforementioned reactive functional groups and in addition bearing at least one functional group selected from among the group constituted of primary-, secondary- and tertiary alkylamine functional groups, quaternary amine functional groups, sulfate-, sulfonate-, phoshate-, phosphonate-, carboxylate-, hydroxyl-, halogen functional groups, and mixtures thereof.

The polymers used in the composition C2 may be selected from among polyethers, polyesters, polyurethanes, polyureas, polyethylene glycols, polypropylene glycols, polyamides, polyacetals, polyimides, polyolefins, polysulphides and polydimethylsiloxanes, the said polymers in addition bearing at least one reactive functional group selected from among the group constituted of the functions: acrylate, methacrylate, vinyl ether, N-vinyl ether, mercaptoester, thiolene, siloxane, epoxy, oxetane, urethane, isocyanate, and peroxide.

Mention of examples of such polymers include, but are not limited to, the following polymers: poly(2-(1-naphthyloxy) ethyl acrylate), poly(2-(2-naphthyloxy) ethyl acrylate), poly(2-(2-naphthyloxy) ethyl methacrylate), polysorbitol dimethacrylate, polyacrylamide, poly((2-(1-naphthyloxy) ethanol), poly(2-(2-naphthyloxy) ethanol), poly(l-chloro-2,3-epoxypropane), poly(n-butyl isocyanate), poly(N-vinyl carbazole), poly(N-vinyl pyrrolidone), poly(p-benzamide), poly(p-chlorostyrene), poly(p-methyl styrene) poly(p-phenylene oxide), poly(p-phenylene sulfide), poly(N-(methacryloxyethyl) succinimide), polybenzimidazole, polybutadiene, polybutylene terephthalate, polychloral, polychloro trifluoro ethylene, polyether imide, polyether ketone, polyether sulfone, polyhydridosilsesquioxane, poly(m-phenylene isophthalamide), poly(methyl 2-acrylamido-2-methoxyacetate), poly(2-acrylamido-2-methylpropanesulfonic acid), poly-mono-butyl maleate, polybutyl methacrylate, poly(N-tert-butylmethacrylamide), poly(N-n-butylmethacrylamide), polycyclohexylmethacrylamide, poly(m-xylenebisacrylamide 2,3-dimethyl-1,3-butadiene, N,N-dimethylmethacrylamide), poly(n-butyl methacrylate), poly(cyclohexyl methacrylate), polyisobutyl methacrylate, poly(4-cyclohexylstyrene), polycyclol acrylate, polycyclol methacrylate, polydiethyl ethoxymethylenemalonate, poly(2,2,2-trifluoroethyl methacrylate), poly(1,1,1-trimethylolpropane trimethacrylate), polymethacrylate, poly(N,N-dimethylaniline dihydrazide), poly(isophthalic dihydrazine), isophthalic polyacid, polydimethyl benzilketal, epichlorohydrin, poly(ethyl-3,3-diethoxyacrylate), poly(ethyl-3,3-dimethylacrylate), poly(ethyl vinyl ketone), poly(vinyl ethyl ketone), poly(penten-3-one), polyformaldehyde, poly(diallyl acetal), polyfumaronitrile, polyglyceryl propoxy triacrylate, polyglyceryl trimethacrylate, polyglycidoxypropyltrimethoxysilane, polyglycidyl acrylate, poly(n-heptyl acrylate), poly(acrylic acid n-heptyl ester), poly(n-heptyl methacrylate), poly(3-hydroxypropionitrile), poly(2-hydroxypropyl acrylate), poly(2-hydroxypropyl methacrylate), poly(N-(methacryloxyethyl) phthalimide), poly(1,9-nonanediol diacrylate), poly(1,9-nonanediol dimethacrylate), poly(N-(n-propyl) acrylamide), poly(ortho-phthalic acid), poly(iso-phthalic acid), poly(1,4-benzenedicarboxylic acid), poly(1,3-benzenedicarboxylic acid), poly(phthalic acid), poly(mono-2-acryloxyethyl ester), terephthalic polyacid, phthalic polyanhydride, polyethylene glycol diacrylate, polyethylene glycol methacrylate, polyethylene glycol dimethacrylate, poly(isopropyl acrylate), polysorbitol pentaacrylate, polyvinyl bromoacetate, polychloroprene, poly(di-n-hexyl silylene), poly(di-n-propyl siloxane), polydimethyl silylene, polydiphenyl siloxane, polyvinyl propionate, polyvinyl triacetoxysilane, polyvinyl tris-tert-butoxysilane, polyvinyl butyral, polyvinyl alcohol, polyvinyl acetate, polyethylene co-vinyl acetate, poly(bisphenol-A polysulfone), poly(1,3-dioxepane), poly(1,3-dioxolane), poly(1,4-phenylene vinylene), poly(2,6-dimethyl-1A-phenylene oxide), poly(4-hydroxybenzoic acid), poly(4-methyl pentene-1), poly(4-vinylpyridine), polymethylacrylonitrile, polymethylphenylsiloxane, polymethylsilmethylene, polymethylsilsesquioxane, poly(phenylsilsesquioxane) poly(pyromellitimide-1,4-diphenyl ether), polytetrahydrofuran, polythiophene, poly(trimethylene oxide), polyacrylonitrile, polyether sulfone, polyethylene-co-vinyl acetate, poly(perfluoroethylene propylene), poly(perfluoroalkoxyl alkane), or poly(styrene)acrylonitrile).

The term “crosslinking agent” is used to refer to a compound bearing at least two reactive functional groups that are capable of crosslinking a monomer or a polymer, or a mixture of monomers or polymers, during its polymerisation.

The crosslinking agent may be selected from among molecules bearing at least two functional groups selected from among the group constituted of the functions: acrylate, methacrylate, vinyl ether, N-vinyl ether, mercaptoester, thiolene, siloxane, epoxy, oxetane, urethane, isocyanate, and peroxide.

By way of crosslinking agent, mention may be made in particular of:

-   -   diacrylates, such as 1,6-hexanediol diacrylate, 1,6-hexanediol         dimethacrylate, polyethylene glycol dimethacrylate,         1,9-nonanediol dimethacrylate, 1,4-butanediol dimethacrylate,         2,2-bis(4-methacryloxyphenyl) propane, 1,3-butanediol         dimethacrylate, 1,10-decanediol dimethacrylate,         bis(2-methacryloxyethyl) N,N′-1,9-nonylene biscarbamate,         1,4-butanediol diacrylate, ethylene glycol diacrylate,         1,5-pentanediol dimethacrylate, 1,4-phenylene diacrylate, allyl         methacrylate, N,N′-methylenebisacrylamide,         2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]propane,         tetraethylene glycol diacrylate, ethylene glycol dimethacrylate,         diethylene glycol diacrylate, triethylene glycol diacrylate,         triethylene glycol dimethacrylate, polyethylene glycol         diglycidyl ether, N,N-diallylacrylamide,         2,2-bis[4-(2-acryloxyethoxy) phenyl]propane, glycidyl         methacrylate;     -   multifunctional acrylates such as dipentaerythritol         pentaacrylate, 1,1,1-trimethylolpropane triacrylate,         1,1,1-trimethylolpropane trimethacrylate, ethylenediamine         tetramethacrylate, pentaerythritol triacrylate, pentaerythritol         tetraacrylate;     -   acrylates also having other reactive functional groups, such as         propargyl methacrylate, 2-cyanoethyl acrylate, tricyclodecane         dimethanol diacrylate, hydroxypropyl methacrylate,         N-acryloxysuccinimide, N-(2-hydroxypropyl) methacrylamide, N-(3         aminopropyl) methacrylamide hydrochloride,         N-(t-BOC-aminopropyl)methacrylamide, 2-am inoethyl methacrylate         hydrochloride, monoacryloxyethyl phosphate, o-nitrobenzyl         methacrylate, acrylic anhydride, 2-(tert-butylamino) ethyl         methacrylate N,N-diallylacrylamide, glycidyl methacrylate,         2-hydroxyethyl acrylate,         4-(2-acryloxyaheoxy)-2-hydroxybenzophenone,         N-(Phthalimidomethyl) acrylamide, cinnamyl methacrylate.

The term “photoinitiator” is used to refer to a compound that is capable of fragmenting under the effect of light radiation.

The photoinitiators which may be used according to the present invention are known in the state of the art and are described, for example in “Les photoinitiateurs dans la réticulation des revêtements [Photoinitiators in the crosslinking of coatings]”, G. Li Bassi, Double Liaison—Chimie des Peintures [Double Bond—Chemistry of Paints], no 361, November 1985, p. 34-41; “Applications industrielles de la polymérisation photoinduite [Industrial applications of photoinduced polymerisation]”, Henri Strub, L'Actualité Chimique [Chemical News], February 2000, p. 5-13; and “Photopolymères: considerations théoriques et réaction de prise [Photopolymers: theoretical considerations and setting reaction”, Marc, J M Abadie, Double Liaison—Chimie des Peintures [Double Bond—Chemistry of Paints], no 435-436, 1992, p. 28-34.

These photoinitiators include:

-   -   α-hydroxyketones, such as         2-hydroxy-2-methyl-1-phenyl-1-propanone, marketed for example         under the trade names DAROCUR® 1173 and 4265, IRGACURE® 184,         2959, and 500, by the company BASF, and ADDITOL® CPK by the         company CYTEC;     -   α-amino ketones, in particular         2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,         marketed, for example, under the trade names IRGACURE® 907 and         369 by the company BASF;     -   aromatic ketones marketed for example under the trade name         ESACURE® TZT by LAMBERTI; or even thioxanthones marketed for         example under the trade name ESACURE® ITX by LAMBERTI, and         quinones. These aromatic ketones most often require the presence         of a hydrogen donor compound such as tertiary amines and in         particular alkanolamines. In particular mention may be made of         the tertiary amine ESACURE® EDB marketed by the company         LAMBERTI.     -   α-dicarbonyl derivatives of which the most common representative         is benzyldimethylketal marketed under the trade name IRGACURE®         651 by the company BASF. Other commercially available products         are marketed by the company LAMBERTI under the trade name         ESACURE® KB1, and     -   acylphosphine oxides, such as, for example, bis-acylphosphine         oxides (BAPO) marketed for example under the trade names         IRGACURE® 819, 1700, and 1800, DAROCUR® 4265, LUCIRIN® TPO, and         LUCIRIN® TPO-L by the company BASF.

Among the photoinitiators, mention may also be made of aromatic ketones such as benzophenone, phenylglyoxylates, such as the methyl ester of phenylglyoxylic acid, oxime esters, such as [1-(4-phenylsulfanylbenzoyl) heptylideneamino] benzoate, sulphonium salts, iodonium salts and oxime sulphonates.

According to one embodiment, the composition C2 may further comprise an additional monomer or polymer capable of improving the properties of the enveloping shell of the microcapsules and/or of contributing new properties to the enveloping shell of the microcapsules.

Among these additional monomers or polymers, mention may be made of monomers or polymers bearing a group that is sensitive to pH, temperature, UV or IR.

These additional monomers or polymers are able to induce the rupture of the solid microcapsules and as a result the subsequent release of their contents, after stimulation via pH, temperature, UV or IR.

These additional monomers or polymers may be selected from among monomers or polymers bearing at least one reactive functional group selected from among the group constituted of the functions: acrylate, methacrylate, vinyl ether, N-vinyl ether, mercaptoester, thiolene, siloxane, epoxy, oxetane, urethane, isocyanate and peroxide, and also bearing one of the following groups:

-   -   a hydrophobic group such as a fluorinated group, for example         trifluoroethyl methacrylate, trifluoroethyl acrylate,         tetrafluoropropyl methacrylate, pentafluoropropyl acrylate,         hexafluorobutyl acrylate, or fluorophenyl isocyanate;     -   a pH sensitive group such as primary, secondary or tertiary         amines, carboxylic acids, phosphate groups, sulfate groups,         nitrate groups, or carbonate groups;     -   a UV-sensitive or UV-cleavable group (or photochromic group)         such as azobenzene, spiropyran, 2-diazo-1,2-naphthoquinone,         o-nitrobenzyl, thiol, or 6-nitro-veratroyloxycarbonyl, for         example poly(ethylene         oxide)-block-poly(2-nitrobenzylmethacrylate), and other block         copolymers, as described in particular in Liu et al., Polymer         Chemistry 2013, 4, 3431-3443;     -   an IR-sensitive or IR-cleavable group such as o-nitrobenzyl or         2-diazo-1,2-naphthoquinone, for example the polymers described         in Liu et al., Polymer Chemistry 2013, 4, 3431-3443; and     -   a temperature sensitive group such as poly         (N-isopropylacrylamide).

Step c)

The step c) of the method according to the invention consists in preparing a second emulsion (E2).

The second emulsion consists of a dispersion of droplets of the first emulsion in a composition C3 that is immiscible with C2, created by addition dropwise of the emulsion (E1) into C3 under agitation.

During the step c), the emulsion (E1) is at a temperature comprised between 15° C. and 60° C. During the step c), the composition C3 is at a temperature comprised between 15° C. and 60° C.

Under the addition conditions of step c), the compositions C2 and C3 are not miscible with each other, which signifies that the amount (by weight) of the composition C2 capable of being solubilised in the composition C3 is less than or equal to 5%, preferably less than 1%, and preferentially less than 0.5%, in relation to the total weight of the composition C3, and that the amount (by weight) of the composition C3 capable of being solubilised in the composition C2 is less than or equal to 5%, preferably less than 1%, and preferentially less than 0.5%, in relation to the total weight of the composition C2.

Thus, when the emulsion (E1) comes into contact with the composition C3 under agitation, the latter is dispersed in the form of droplets, referred to as double droplets, the dispersion of these droplets of emulsion (E1) in the continuous phase C3 being referred to as emulsion (E2).

Typically, a double droplet formed during the step c) corresponds to a single droplet of the composition C1 as described here above, encased by an enveloping shell of composition C2 which completely encapsulates the said single droplet.

The double droplet formed during the step c) may also comprise at least two single droplets of the composition C1, the said single droplets being encased by an enveloping shell of composition C2 which completely encapsulates the said single droplets.

Thus, the said double droplets comprise a core constituted of one or more single droplets of the composition C1, and a layer of the composition C2 encasing the said core.

The resulting emulsion (E2) is generally a polydisperse double emulsion (C1-in-C2-in-C3 emulsion or C1/C2/C3 emulsion), which signifies that the double droplets do not have a clear size distribution in the emulsion (E2).

The immiscibility between the compositions C2 and C3 provides the ability to prevent mixing between the layer of the composition C2 and the composition C3 and thus ensures the stability of the emulsion (E2).

The immiscibility between the compositions C2 and C3 also provides the ability to prevent the water-soluble or hydrophilic substance of the composition C1 from migrating from the core of the droplets to the composition C3.

In order to implement the step c), use may be made of any type of agitator usually used to form emulsions, such as, for example, a mechanical agitator with paddles, a static emulsifier, an ultrasonic homogeniser, a membrane homogeniser, a high pressure homogeniser, a colloid mill, a high shear disperser or a high speed homogeniser.

Composition C3

According to one embodiment, the viscosity of the composition C3 at 25° C. is greater than the viscosity of the emulsion (E1) at 25° C.

According to the invention, the viscosity of the composition C3 at 25° C. is comprised between 500 mPa·s and 100,000 mPa·s.

Preferably, the viscosity of the composition C3 at 25° C. is comprised between 3,000 mPa·s and 100,000 mPa·s, preferentially between 5,000 mPa·s and 80,000 mPa·s, for example between 7,000 mPa·s and 70,000 mPa·s.

According to this embodiment, given the very high viscosity of the continuous phase formed by the composition C3, the rate of destabilisation of the double droplets of the emulsion (E2) is significantly slow in relation to the duration of the method of the invention, which thus then provides for kinetic stabilisation of the emulsions (E2) and then (E3) until such time as the polymerisation of the enveloping shell of the capsules is completed. The capsules once polymerised are thermodynamically stable.

Thus, the very high viscosity of the composition C3 ensures the stability of the emulsion (E2) obtained at the conclusion of step b).

The high viscosity of the composition C3 make it possible to advantageously ensure the kinetic stability of the double emulsion (E2), thus preventing it from undergoing phase separation (dephasing) over the duration of the manufacturing method.

Preferably, the interfacial tension between the compositions C2 and C3 is low. The low interfacial tension between the compositions C2 and C3 also advantageously makes it possible to ensure the stability of the emulsion (E2) obtained at the conclusion of the step c).

The volume fraction of the first emulsion (E1) in C3 can be varied from 0.05 to 0.5 in order, on the one hand, to improve the production yield and, on the other hand, to cause to vary the mean diameter of the capsules. At the end of this step, the size distribution of the second emulsion is relatively wide.

According to one embodiment, the ratio between the volume of emulsion (E1) and the volume of composition C3 varies between 1:10 and 10:1. Preferably, this ratio is comprised between 1:9 and 3:1, preferentially between 1:9 and 1:1.

According to one embodiment, the composition C3 comprises in addition at least one branched polymer, preferably having a molecular weight greater than 5000 g·mol⁻¹, and/or at least one polymer having a molecular weight greater than 5000 g·mol⁻¹, and/or solid particles such as silicates.

According to one embodiment, the composition C3 comprises at least one branched polymer, preferably having a molecular weight greater than 5000 g·mol⁻¹, preferentially between 10,000 g·mol⁻¹ and 500,000 g·mol⁻¹, for example between 50,000 g·mol⁻¹ and 300,000 g·mol⁻¹.

The term “branched polymer” is used to refer to a polymer having at least one branch point between its two end groups, a branch point being a point of a chain on which is attached a side chain also referred to as branch or pendant chain.

Among the branched polymers, mention may be made for example of graft polymers, comb polymers, or indeed star polymers or dendrimers.

According to one embodiment, the composition C3 comprises at least one polymer having a molecular weight greater than 5000 g·mol⁻¹, preferentially between 10,000 g·mol⁻¹ and 500,000 g·mol⁻¹, for example between 50,000 g·mol⁻¹ and 300,000 g·mol⁻¹.

By way of a polymer that may be used in the composition C3, mention may be made of the following compounds, used alone or indeed mixed together:

-   -   cellulose derivatives, such as cellulose ethers: methyl         cellulose, ethyl cellulose, hydroxyethyl cellulose,         methylhydroxyethyl cellulose, ethylhydroxyethyl cellulose,         carboxymethyl cellulose, hydroxypropyl cellulose, or         methylhydroxypropyl cellulose;     -   polyacrylates (also known as carbomers), such as polyacrylic         acid (PAA), polymethacrylic acid (PMAA), poly(hydroxyethyl         methacrylate) (pHEMA), poly(N-2-hydroxypropyl methacrylate)         (pHPMA);     -   polyacrylamides such as poly(N-isopropylacrylamide) (PNIPAM);     -   polyvinylpyrrolidone (PVP) and the derivatives thereof;     -   polyvinyl alcohol (PVA) and the derivatives thereof;     -   poly(ethylene glycol), poly(propylene glycol) and the         derivatives thereof, such as poly(ethylene glycol)         acrylate/methacrylate, poly(ethylene glycol)         diacrylate/dimethacrylate, polypropylene carbonate;     -   polysaccharides such as carrageenans, carob gums or tara gums,         dextran, xanthan gums, chitosan, agarose, hyaluronic acids,         gellan gum, guar gum, arabic gum, tragacanth gum, diuretic gum,         oat gum, karaya gum, ghatti gum, curdlan gum, pectin, konjac         gum, starch;     -   protein derivatives such as gelatin, collagen, fibrin,         polylysine, albumin, casein;     -   silicone derivatives such as polydimethylsiloxane (also known as         dimethicone), alkyl silicones, aryl silicones, alkyl aryl         silicones, polyethylene glycol dimethicones, polypropylene         glycol dimethicone;     -   waxes, such as diester waxes (alkanediol diesters, hydroxyl acid         diesters), triester waxes (triacylglycerols; triesters of         alkane-1,2-diol, w-hydroxy acid and fatty acid; esters of         hydroxymalonic acid, fatty acid and alcohol; triesters of         hydroxyl acids, fatty acid and fatty alcohol, triesters of fatty         acid, hydroxyl acid and diol) and polyester waxes (polyesters of         fatty acids). The fatty acid esters which may be used by way of         waxes in the context of the invention are, for example, cetyl         palmitate, cetyl octanoate, cetyl laurate, cetyl lactate, cetyl         isononanoate, and cetyl stearate, stearyl stearate, myristyl         stearate, cetyl myristate, isocetyl stearate, glyceryl         trimyristate, glyceryl tripalmitate, glyceryl monostearate, or         glyceryl palmitate and cetyl palmitate;     -   fatty acids which may be used as waxes such as cerotic acid,         palmitic acid, stearic acid, dihydroxystearic acid, behenic         acid, lignoceric acid, arachidic acid, myristic acid, lauric         acid, tridecyclic acid, pentadecyclic acid, margaric acid,         nonadecyclic acid, henicosylic acid, tricosylic acid,         pentacosylic acid, heptacosylic acid, montanic acid or         nonacosylic acid;     -   fatty acid salts, in particular fatty acid aluminum salts such         as aluminum stearate, hydroxyl aluminum bis (2-ethylhexanoate);     -   isomerised jojoba oil;     -   hydrogenated sunflower oil;     -   hydrogenated coconut oil;     -   hydrogenated lanolin oil;     -   castor oil and the derivatives thereof, in particular modified         hydrogenated castor oil or compounds obtained by esterification         of castor oil with fatty alcohols;     -   polyurethanes and the derivatives thereof;     -   styrenic polymers such as styrene butadiene;     -   polyolefins such as polyisobutene.

According to one embodiment, the composition C3 comprises solid particles such as clays, silicas and silicates.

By way of solid particles that may be used in the composition C3, mention may be made of clays and silicates belonging in particular to the category of phyllosilicates (also known as layered silicates). By way of example of a silicate that may be used in the context of the invention, mention may be made of Bentonite, Hectorite, Attapulgite, Sepiolite, Montmorillonite, Saponite, Sauconite, Nontronite, Kaolinite, Talc, Sepiolite, Chalk. The fumed synthetic silicas may also be used. The clays, silicates and silicas previously mentioned above can advantageously be modified by organic molecules such as polyethers, ethoxylated amides, quaternary ammonium salts, long-chain diamines, long-chain esters, polyethylene glycols, polypropylene glycols.

These particles may be used either alone or mixed together.

According to one embodiment, the composition C3 comprises at least one polymer having a molecular weight greater than 5000 g·mol⁻¹ and solid particles. Any mixture of the compounds previously mentioned above may be used.

Step d)

The step d) of the method according to the invention consists in refining the size of the droplets of the second emulsion (E2).

This step may consist in applying a homogeneous controlled shear to the emulsion (E2), with the said shear rate applied comprised between 10 s⁻¹ and 100,000 s⁻¹.

According to one embodiment, the polydisperse double droplets obtained in the step c) are subjected to a size refinement process consisting of subjecting them to shearing capable of fragmenting them into new double droplets of homogeneous and controlled diameters. Preferably, this fragmentation step is carried out by using a high-shear cell such as the Couette type cell according to a method described in the patent application EP 15 306 428.2.

According to one embodiment, in the step d), the second emulsion (E2), obtained at the conclusion of the step c), consisting of polydisperse double droplets dispersed in a continuous phase, is subjected to a shear in a mixer, which applies a homogeneous controlled shear.

Thus, according to this embodiment, the step d) consists in applying a homogeneous controlled shear to the emulsion (E2), with the said shear rate applied comprised between 1000 s⁻¹ and 100,000 s⁻¹.

According to this embodiment, in a mixer, the shear rate is said to be controlled and homogeneous, regardless of the duration of application, when it passes to a maximum value that is identical for all the parts of the emulsion, at a given time instant that may vary from one point of the emulsion to another. According to the invention, the exact configuration of the mixer is not critical as long as the entire emulsion has been subjected to the same maximum shear when exiting from this device. The mixers that are suitable for carrying out the step d) are described in particular in the document U.S. Pat. No. 5,938,581.

The second emulsion may undergo homogeneous controlled shear when it circulates through a cell formed by:

-   -   two concentric rotary cylinders (also referred to as a Couette         type mixer);     -   two parallel rotating discs; or     -   two parallel oscillating plates.

According to this embodiment, the shear rate applied to the second emulsion is comprised between 1,000 s⁻¹ and 100,000 s⁻¹, preferably between 1,000 s⁻¹ and 50,000 s⁻¹, and preferentially between 2,000 s⁻¹ and 20,000 s⁻¹.

According to this embodiment, during the step d), the second emulsion is introduced into the mixer and is then subjected to shear which results in the formation of the third emulsion. The third emulsion (E3) is chemically identical to the second emulsion (E2) but consists of monodisperse double droplets while the emulsion (E2) consists of polydisperse double droplets. The third emulsion (E3) typically consists of a dispersion of double droplets comprising a core constituted of one or more droplets of the composition C1 and a layer of the composition C2 encapsulating the said core, the said double droplets being dispersed in the composition C3.

The difference between the second emulsion and the third emulsion is the size variance of the double droplets: the droplets of the second emulsion are polydisperse in size while the droplets of the third emulsion are monodisperse, thanks to the fragmentation mechanism described here above.

Preferably, according to this embodiment, the second emulsion is introduced in a continuous manner into the mixer, which signifies that the quantity of double emulsion (E2) introduced at the inlet of the mixer is the same as the quantity of third emulsion (E3) at the outlet of the mixer.

Given that the size of the droplets of the emulsion (E3) corresponds essentially to the size of the droplets of the solid microcapsules after polymerisation, it is possible to adjust the size of the microcapsules and the thickness of the enveloping shell by adjusting the shear rate during the step d), with a strong correlation between the decrease in the size of droplets and the increase in shear rate. This makes it possible to adjust the resulting dimensions of the microcapsules by varying the shear rate applied during the step d).

According to one preferred embodiment, the mixer implemented during the step d) is a Couette type mixer, comprising two concentric cylinders, one external cylinder having internal radius Ro, and an internal cylinder having external radius Ri, the external cylinder being fixed and the internal cylinder being in rotation with an angular velocity ω.

A Couette-type mixer that is appropriate for the method of the invention may be supplied by the company TSR France.

According to one embodiment, the angular velocity ω of the internal rotating cylinder of the Couette type mixer is greater than or equal to 30 rad·s⁻¹.

For example, the angular velocity ω of the internal rotating cylinder of the Couette type mixer is about 70 rad·s⁻¹.

The dimensions of the fixed external cylinder of the Couette type mixer may be chosen so as to modulate the space (d=Ro−Ri) between the rotating internal cylinder and the fixed external cylinder.

According to one embodiment, the space (d=Ro−Ri) between the two concentric cylinders of the Couette type mixer is comprised between 50 μm and 1000 μm, preferably between 100 μm and 500 μm, for example between 200 μm and 400 μm.

For example, the distance d between the two concentric cylinders is equal to 100 μm.

According to this embodiment, during the step d), the second emulsion is introduced at the inlet of the mixer, typically via a pump, and is directed towards the space between the two concentric cylinders, the external cylinder being fixed and the internal cylinder being in rotation at an angular velocity ω.

When the double emulsion is in the space between the two cylinders, the shear rate applied to the said emulsion is given by the following equation:

$\gamma = \frac{R_{i}\omega}{\left( {R_{o} - R_{i}} \right)}$

wherein:

-   -   ω is the angular velocity of the rotating internal cylinder;     -   Ro is the internal radius of the fixed external cylinder; and     -   Ri is the external radius of the internal rotating cylinder.

According to another embodiment, when the viscosity of the composition C3 is greater than 2000 mPa·s at 25° C., the step d) consists in applying to the emulsion (E2) a shear rate of less than 1000 s⁻¹.

According to this embodiment, the fragmentation step d) may be carried out by making use of any type of mixer usually used to form emulsions with a shear rate of less than 1000 s⁻¹, in which case the viscosity of the composition C3 is greater than 2000 mPa·s, that is to say, under conditions such as those described in the patent application FR 16 61787.

The geometric characteristics of the double droplets formed at the end of this step will dictate those of the future capsules.

According to this embodiment, in the step d), the emulsion (E2), constituted of polydisperse droplets dispersed in a continuous phase, is subjected to shear, for example in a mixer, at a low shear rate, that is to say less than 1000 s⁻¹.

According to this embodiment, the shear rate applied in the step d) is for example comprised between 10 s⁻¹ and 1000 s⁻¹.

Preferably, the shear rate applied in the step d) is strictly less than 1000 s⁻¹.

According to this embodiment, the droplets of emulsion (E2) can be efficiently fragmented into fine and monodisperse droplets of emulsion (E3) only if a high shear stress is applied thereto.

The shear stress σ applied to a droplet of emulsion (E2) is defined as the tangential force per unit surface area of droplet resulting from the macroscopic shear applied to the emulsion during agitation thereof during the step d).

The shear stress σ (expressed in Pa), the viscosity of the composition C3 q (expressed in Pa s) and the shear rate γ (expressed in s⁻¹) applied to the emulsion (E2) during agitation thereof during the course of step d) are linked by the following equation:

σ=θγ

Thus, according to this embodiment, the high viscosity of the composition C3 makes it possible to apply a very high shear stress to the droplets of emulsion (E2) in the mixer, even if the shear rate is low and the shear inhomogeneous.

In order to implement the step d) according to this embodiment, use may be made of any type of agitator usually used to form emulsions, such as, for example, a mechanical agitator with paddles, a static emulsifier, an ultrasonic homogeniser, a membrane homogeniser, a high pressure homogeniser, a colloid mill, a high shear disperser or a high speed homogeniser.

According to one preferred embodiment, use is made of a simple emulsifier such as a mechanical paddle agitator or a static emulsifier in order to implement the step d). Indeed, this is possible because this embodiment requires neither controlled shear nor shear force greater than 1000 s⁻¹.

Step e)

The step e) of the method of the invention consists of the crosslinking and therefore the formation of the enveloping shell of the solid microcapsules according to the invention.

This step makes it possible both to achieve the expected protection and retention performance of the capsules and to ensure the thermodynamic stability thereof, thereby definitively preventing any destabilisation mechanism such as coalescence or curing.

According to one embodiment, when the composition C2 comprises a photoinitiator, the step e) is a step of photopolymerisation consisting in exposing the emulsion (E3) to a light source that is capable of initiating the photopolymerisation of the composition C2, in particular to a UV light source emitting preferably in the wavelength range of between 100 nm and 400 nm, and this being in particular for a time period of less than 15 minutes.

According to this embodiment, the step e) consists in subjecting the emulsion (E3) to photopolymerisation, which will thus enable the photopolymerisation of the composition C2. This step will provide the ability to obtain microcapsules that encapsulate the water-soluble substance as defined here above.

According to one embodiment, the step e) consists in exposing the emulsion (E3) to a light source capable of initiating the photopolymerisation of the composition C2.

Preferably, the light source is a source of UV light.

According to one embodiment, the UV light source emits in the wavelength range of between 100 nm and 400 nm.

According to one embodiment, the emulsion (E3) is exposed to a light source for a period less than 15 minutes, and preferably for 5 to 10 minutes.

During the step e), the enveloping shell of the aforementioned double droplets, constituted of the photocrosslinkable composition C2, is cross-linked and thus converted into a viscoelastic polymeric enveloping shell, that encapsulates and protects the water-soluble substance from being released in the absence of a mechanically triggered mechanism.

According to one other embodiment, when the composition C2 does not comprise a photoinitiator, the step e) is a polymerisation step, without exposure to a light source, with the duration of this step e) of polymerisation preferably being comprised between 8 hours and 100 hours and/or this step e) being carried out at a temperature comprised between 20° C. and 80° C.

According to this embodiment, the polymerisation is initiated for example by exposure to heat (thermal initiation), or simply by bringing about contact of the monomers, polymers and crosslinking agents with each other, or with a catalyst. The time of polymerisation is then generally greater than several hours.

Preferably, the step e) of polymerisation of the composition C2 is carried out for a time period of between 8 hours and 100 hours, at a temperature comprised between 20° C. and 80° C.

The composition obtained at the conclusion of step e), comprising of solid microcapsules dispersed in the composition C3, is ready for use and may be used without any additional step of post-treatment of the capsules being required.

The thickness of the enveloping shell of the microcapsules thus obtained is typically comprised between 0.1 μm and 20 μm, preferably between 0.2 μm and 10 μm, preferably between 0.2 μm and 8 μm.

According to one embodiment, the solid microcapsules obtained at the conclusion of the step e) are free of any surfactant, in particular at the level of the interface between the solid enveloping shell and the external medium (or continuous phase) represented by the composition C3.

The method of the invention presents the advantage of not requiring a surfactant, in any of the steps b) to e) of formation of the enveloping shell of the solid microcapsules. The method of the invention thus makes it possible to reduce the presence of additives which could modify the properties of the final product obtained after release of the water-soluble substance.

The present invention also relates to a series (or set) of solid microcapsules, which it is possible to obtain in accordance with the method as defined here above, in which each microcapsule includes:

-   -   a core comprising of a composition C1 as defined here above; and     -   a solid enveloping shell that completely encapsulates at its         periphery the core,

in which the mean diameter of the said microcapsules is comprised between 1 μm and 30 μm, the thickness of the rigid enveloping shell is comprised between 0.1 μm and 20 μm, and the standard deviation of the distribution of the diameter of microcapsules is less than 50%, in particular less than 25%, or less than 1% μm.

As indicated here above, the method of the invention makes it possible to obtain monodisperse particles. Also, the series of solid microcapsules mentioned here above is made up of a population of particles that are monodisperse in size. Thus, the standard deviation of the distribution of the diameter of microcapsules is less than 50%, in particular less than 25%, or less than 1 μm.

The distribution of size of the solid microcapsules can be measured by means of the light scattering technique using a Mastersizer 3000 (Malvern Instruments) equipped with a Hydro SV measurement cell.

According to one embodiment, the aforementioned solid microcapsules comprise a solid enveloping shell entirely composed of crosslinked polymer (obtained from the composition C2).

As indicated here above, the method of the invention makes it possible to obtain solid microcapsules. The present invention therefore also relates to solid microcapsules comprising of a core and a solid enveloping shell that completely encapsulates at its periphery the core, wherein the core is a composition C1 as defined here above, dispersed in a hydrophobic phase, and wherein the said rigid enveloping shell is constituted of crosslinked polymer,

with the diameter of the said microcapsule being comprised between 1 μm and 30 μm and the thickness of the rigid enveloping shell being comprised between 0.1 μm and 20 μm.

The present invention also relates to a composition comprising of a series of solid microcapsules as defined here above.

The capsules of the invention may advantageously be used in order to protect the active ingredients used in the formulations of polymer, elastomer, rubber, paint, adhesive, sealant, mortar, paper, varnish or coating; in products of synthetic chemistry; in cleaning/washing products, detersive products, laundry products and home care products; in agrochemical products such as fertilisers, herbicides, insecticides, fungicides or pesticides; textiles; petrochemical products such as lubricants, fuels, bitumens, drilling fluids and well stimulation fluids.

The present invention also relates to a method for releasing a water-soluble or hydrophilic substance, in particular in solid form at ambient temperature and atmospheric pressure, in particular in the form of particles and/or in the form of a crystal, in particular of a salt, preferably having an average size of less than 1 μm, that includes a step of applying a mechanical shear stress to a composition comprising of a series of solid microcapsules as defined here above.

The expressions “comprised between . . . and . . . ”, “ranging from . . . to . . . ” and “from . . . to . . . ” are to be understood as inclusive of limits, unless otherwise specified.

The following examples are provided by way of illustrating the present invention without limiting the scope thereof.

EXAMPLES Example 1: Manufacture of Solid Capsules According to the Invention

A mechanical agitator (Ika Eurostar 20) equipped with a deflocculating type propeller stirrer is used to carry out all the stirring/agitation steps.

Step a): Creation of the Dispersion of Particles (Composition C1) by Means of Nanoemulsion

Raw Materials % Composition A Paraffin Oil 48 Abil Em 90 (Cetyl PEG/PPG- 2 10/1 Dimethicone, Evonik) Composition B Calcium Chloride 5 Deionised Water 45 Total 100

-   -   The composition A is placed under agitation at 1000 rotations         per minute (rpm) until complete homogenisation is obtained.     -   The composition B is placed under agitation at 1000 rotations         per minute (rpm) until complete homogenisation is obtained.     -   The composition B is subsequently added dropwise to the         composition A under agitation at 2000 rpm. The agitation is         maintained for a period of 5 minutes after the addition and the         mixture is thereafter subjected to sonication (Vibra-cell 75042,         Sonics) for a period of 3 minutes (pulse 5 s/2 s) for a period         of 3 minutes at 30% amplitude.     -   The nanoemulsion obtained is then placed under reduced pressure         and heated (25 mbar, 40° C.) until complete evaporation of the         water occurs. The composition C1 is thus obtained.

Step b): Preparation of the First Emulsion (E1)

Raw Materials % Composition C2 CN 9800 (Silicone Urethane 92 Acrylate Oligomer, Sartomer) CN381 (Amine Modified Polyether 5 Oligomer, Sartomer) Darocur 1173 (Photoinitiator, 3 BASF) Total 100

The composition C1 is added dropwise to the composition C2 under agitation at 2000 rpm in a ratio of 3:7. This results in the first emulsion (E1).

Step c): Preparation of the Second Emulsion (E2)

Raw Materials % First emulsion — 5 Composition C3 Sodium Alginate (Sigma 9.5 Aldrich) Deionised Water 85.5 Total 100

The composition C3 is placed under agitation at 1000 rotations per minute (rpm) until complete homogenisation is obtained and then allowed to stand for a period of one hour at ambient temperature. The first emulsion (E1) is then added dropwise to the composition C3 under agitation at 1000 rpm. The second emulsion (E2) is thus obtained.

Step d): Size Refinement of the Second Emulsion

The second polydisperse emulsion (E2) obtained in the previous step is agitated at 1000 rpm for a period of 10 minutes. A monodisperse emulsion (E3) is thus obtained.

Step e): Cross-Linking of the Enveloping Shell of the Capsules

The second monodisperse emulsion (E3) obtained in the previous step is irradiated for a period of 10 minutes by using a UV light source (Dymax LightBox ECE 2000) having a maximum luminous intensity of 0.1 W/cm² at a wavelength of 365 nm.

The microcapsules obtained present a good size distribution, that is to say, an average size of 8.7 μm and the size distribution thereof has a standard deviation of 2.1 μm that is 24%.

Moreover, the quality of encapsulation of the calcium chloride with the microcapsules according to Example 1 was studied by monitoring the viscosity of the suspension of capsules for a period of 20 days at ambient temperature. It is found that the viscosity does not undergo any significant increase, indicating the absence of leakage of calcium chloride to the exterior of the capsules where it would cause the gelation of the alginate. The microcapsules discussed in Example 1 thus prove to be particularly suitable for effectively encapsulating a water-soluble substance in the form of particles. 

1. A method for preparing solid microcapsules that comprises of the following steps: a) preparing a composition C1, comprising at least one water-soluble or hydrophilic substance dispersed in a hydrophobic phase; b) adding, under agitation, of said composition C1 in a polymeric composition C2, the compositions C1 and C2 being immiscible with each other; the composition C2 comprising at least one monomer or polymer, at least one crosslinking agent, and optionally at least one photoinitiator or a crosslinking catalyst; the viscosity of the composition C2 being comprised between 500 mPa·s and 100 000 mPa·s at 25° C., wherein an emulsion (E1) is obtained comprising droplets of the composition C1 dispersed in the composition C2; c) adding, under agitation, of the emulsion (E1) in a composition C3, the compositions C2 and C3 being immiscible with each other; the viscosity of the composition C3 being comprised between 500 mPa·s and 100,000 mPa·s at 25° C., and preferably being greater than the viscosity of the emulsion (E1); wherein a double emulsion (E2) is obtained comprising droplets dispersed in the composition C3; d) applying a shear to the emulsion (E2); wherein a double emulsion (E3) is obtained comprising size controlled droplets dispersed in the composition C3; and e) polymerizing the composition C2, wherein solid microcapsules dispersed in the composition C3 are obtained.
 2. The method according to claim 1, in which the water-soluble or hydrophilic substance is in solid form at ambient temperature and atmospheric pressure.
 3. The method according to claim 2, in which the composition C1 is prepared, either by dispersing, in the hydrophobic phase, at least one water-soluble or hydrophilic solid substance in the form of a powder, or by preparing a nanoemulsion of droplets of water in the hydrophobic phase.
 4. The method according to claim 2, in which the composition C1 is obtained by means of grinding at least one solid water-soluble or hydrophilic substance and then dispersing the said ground substance in the hydrophobic phase.
 5. The method according to claim 1, in which the hydrophobic phase comprises at least one oil whose interfacial tension with water is comprised between 25 mN/m and 50 mN/m.
 6. The method according to claim 5, in which the oil is selected from among plant, animal or synthetic oils.
 7. The method according to claim 1, in which the composition C2 comprises from 0.001% to 70% by weight of crosslinking agent(s) in relation to the total weight of said composition.
 8. The method according to claim 1, in which the step d) consists in applying a homogeneous controlled shear to the emulsion (E2), with the said shear rate applied comprised between 1000 s⁻¹ and 100,000 s⁻¹.
 9. The method according to claim 1, in which when the viscosity of the composition C3 is greater than 2000 mPa·s at 25° C., the step d) consists in applying to the emulsion (E2) a shear rate of less than 1000 s⁻¹.
 10. The method according to claim 1, in which, when the composition C2 comprises a photoinitiator, the step e) is a step of photopolymerisation consisting in exposing the emulsion (E3) to a light source that is capable of initiating the photopolymerisation of the composition C2.
 11. The method according to claim 1, in which, when the composition C2 does not comprise a photoinitiator, the step e) is a polymerisation step, without exposure to a light source.
 12. The method according to claim 1, in which the composition C3 comprises in addition at least one branched polymer and/or at least one polymer having a molecular weight greater than 5000 g·mol⁻¹, and/or solid particles.
 13. A series of solid microcapsules, in which each microcapsule includes: a core comprising a composition C1 as defined according to claim 1; and a solid enveloping shell that completely encapsulates at its periphery the core; in which the mean diameter of the said microcapsules is comprised between 1 μm and the thickness of the rigid enveloping shell is comprised between 0.1 μm and 20 μm and the standard deviation of the distribution of the diameter of microcapsules is less than 50%, or less than 1 μm.
 14. A composition comprising a series of solid microcapsules according to claim
 13. 15. A method for releasing a water-soluble or hydrophilic substance, that includes a step of applying a mechanical shear stress to a composition comprising of a series of solid microcapsules according to claim
 14. 